Dc-dc converter, driving apparatus having dc-dc converter, and display device having driving apparatus

ABSTRACT

A DC-DC converter includes a main inductor connected to an input voltage, a main switching element connected in series to the main inductor, a main diode connected to the main inductor and a load, and a main capacitor connected to the main diode and the load, the DC-DC converter changing the input voltage to output a changed input voltage as an output voltage. The DC-DC converter further includes an oscillator connected between the main inductor and the main diode, an auxiliary switching element connected to the oscillator, the auxiliary switching element changing an operation state based on an externally applied control signal to control the oscillator, and a diode unit connected to the oscillator and the auxiliary switching element and controlling current flow based on operations of the oscillator and the auxiliary switching element to change the output voltage. The main switching element and the main diode are to be zero voltage switching or zero current switching in accordance with the operations of the oscillator and the auxiliary switching element.

This application claims priority to Korean Patent Application No.10-2004-0059239, filed on Jul. 28, 2004 and all the benefits accruingtherefrom under 35 U.S.C. § 119, and the contents of which in itsentirety are herein incorporated by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a display device, a driving apparatusof a light source for the display device, and a DC-DC converter.

(b) Description of the Related Art

Display devices used for monitors of computers and television setsinclude self-emitting displays such as light emitting diodes (“LEDs”),electroluminescence (“EL”) devices, vacuum fluorescent displays(“VFDs”), field emission displays (“FEDs”), and plasma panel displays(“PDPs”), and non-emitting displays such as liquid crystal displays(“LCDs”) requiring a light source.

An LCD includes two panels provided with field-generating electrodes anda liquid crystal (“LC”) layer with dielectric anisotropy interposedtherebetween. The field-generating electrodes are supplied with electricvoltages and generate an electric field in the LC layer, and thetransmittance of light passing through the panels varies depending onthe strength of the applied field, which can be controlled by theapplied voltages. Accordingly, desired images are obtained by adjustingthe applied voltages.

The light may be emitted from a light source equipped in the LCD or maybe natural light.

A lighting device for an LCD, i.e., a backlight unit, usually includes aplurality of fluorescent lamps such as external electrode fluorescentlamps (“EEFLs”) and cold cathode fluorescent lamps (“CCFLs”), or aplurality of light emitting diodes (“LEDs”), as light sources, whichuniformly transmit the light to the entire front surface of the LCpanels from the rear thereof.

When using the fluorescent lamps, characteristics of elements of thedisplay device are deteriorated due to large power consumption andheating.

In addition, the fluorescent lamps have a stick shape that aresusceptible to breakage on impact. Moreover, since temperatures of thelamps vary in accordance with positions thereof, to make luminance ofthe lamps vary, image quality of the LCD decreases.

However, when using LEDs, since each LED is a semiconductor device, thelifetime of the LED is long, the lighting speed of the LED is fast, andthe power consumption is low. The LED also withstands impacts well andminiaturization thereof is easy.

Because of these benefits, LEDs are equipped in monitors for middle orlarge sized LCDs such as for computers or television sets, as well as insmall sized LCDs such as in mobile telephones, for a light source.

Fluorescent lamps are activated by an alternate current (“AC”) voltage,but the LEDs are activated by a direct current (“DC”) voltage.Accordingly, for using the LEDs, the backlight unit includes a DC-DCconverter for converting an AC voltage into a DC voltage and changingthe DC voltage to a predetermined magnitude. The DC-DC converter may bea boost converter.

In the DC-DC converter, which includes a semiconductor switching elementand a diode, a switching loss such as a turn-on switching loss orturn-off switching loss occurs in turning-on or turning-off of theswitching element such that power consumption increases.

To decrease the power consumption, a soft switching method is proposed.The soft switching method includes a zero voltage switching type and azero current switching type.

The zero voltage switching type is adapted to majority-carriersemiconductor elements having a large turn-on switching loss, and thezero current switching type is adapted to minority-carriersemiconductors having a large turn-off switching loss.

However, although the soft switching method is used, voltage and currentstress of the switching element increases due to oscillation by acapacitor and an inductor such that switching availability is decreasedand a conduction loss is generated. The soft switching method still hascurrent stress due to a reverse recovery characteristic, which is areverse current known as “a diode reverse recovery current” that flowsin the diode for discharging charges charged in the diode in turning-onof the diode.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the present invention, a DC-DC converter including amain inductor connected to an input voltage, a main switching elementconnected in series to the main inductor, a main diode connected to themain inductor and a load, and a main capacitor connected to the maindiode and the load, the DC-DC converter changing the input voltage tooutput a changed input voltage as an output voltage is provided, andfurther includes an oscillator connected between the main inductor andthe main diode, an auxiliary switching element connected to theoscillator, of which an operation state is changed based on anexternally applied control signal to control the oscillator, and a diodeunit connected to the oscillator and the auxiliary switching element andcontrolling current flow based on operations of the oscillator and theauxiliary switching element to change the output voltage, wherein themain switching element and the main diode are to be zero voltageswitching or zero current switching in accordance with the operations ofthe oscillator and the auxiliary switching element.

The oscillator may include a first capacitor connected in parallel tothe main switching element, a first inductor connected between the firstcapacitor and the main diode, and a second inductor connected betweenthe first capacitor and the auxiliary switching element.

The main switching element may be a semiconductor device of a typedifferent from that of the auxiliary switching element.

The auxiliary switching element may be a metal oxide silicon fieldeffect transistor (“MOSFET”).

The diode unit may include a first diode connected to the oscillator andthe auxiliary switching element and controlling the current flow basedon the operations of the oscillator and the auxiliary switching elementand a second diode connected to the oscillator and the auxiliaryswitching element and controlling a current flow from the oscillatorinto the load based on an operation of the auxiliary switching element

The second diode may be a diode for clamping.

In a further embodiment of the present invention, a driving apparatus ofa display device including a light source unit having a plurality oflight sources, the driving apparatus is provided, and further includes apower supply applying a driving voltage for driving the light sources,and a control signal outputting a first control signal controlling thepower supply and a second control signal controlling the light sources,the driving apparatus controlling operations of the light sources by thedriving voltage based on the second control signal, wherein the powersupply comprises a DC-DC converter having a main switching element ofwhich operation is changed by the first control signal, a main diodeconnected to the light source unit, an oscillator connected between themain switching element and the main diode, and an auxiliary switchingelement connected to the oscillator and controlling operation of theoscillator, the DC-DC converter boosting an input DC voltage to thedriving voltage with a predetermined magnitude, and applying the boosteddriving voltage to the light source unit, wherein the DC-DC converter isto be zero voltage switching or zero current switching of the mainswitching element and the main diode in accordance with operations ofthe oscillator and the auxiliary switching element.

The oscillator may include a first capacitor connected in parallel tothe main switching element; a first inductor connected to the firstcapacitor, and a second inductor connected between the first capacitorand the auxiliary switching element.

The power supply may further include a rectifier changing an externallysupplied AC voltage into a DC voltage and a smoother smoothing the DCvoltage from the rectifier and outputting the smoothed DC voltage as theinput DC voltage.

The power supply may further include a filter filtering noises includedin the AC voltage.

The DC-DC converter may further include a third inductor connected tothe input DC voltage, a first diode connected to the auxiliary switchingelement and the first inductor, a second diode connected to the secondinductor, and a second capacitor connected to the main diode and thesecond diode.

The main switching element may be a semiconductor device of a typedifferent from that of the auxiliary switching element.

The auxiliary switching element may be a metal oxide silicon fieldeffect transistor (“MOSFET”).

The light sources may include LEDs (light emitting diodes) of at leastone red, green, and blue colors.

In a still further embodiment of the present invention, a DC-DCconverter includes a first inductor connected to an input end, a firstswitching element connected to the first inductor and the input end, afirst diode connected between the first inductor and an output end, afirst capacitor connected between the first diode and the output end, anoscillating unit connected between the first inductor and the firstdiode, a second switching element connected to the oscillating unit, asecond diode connected between the second switching element and theinput end, and a third diode connected between the oscillating unit andthe first diode.

The oscillating unit may include a second capacitor connected to thefirst inductor, a second inductor connected between the second capacitorand the second diode, and a third inductor connected between the secondcapacitor and the second switching element.

The first switching element may be a semiconductor device of a typedifferent from that of the second switching element.

The second switching element may be a metal oxide silicon field effecttransistor (“MOSFET”).

In a still further embodiment of the present invention, a display deviceincluding a plurality of pixels, a lamp unit emitting light based on alamp control signal, and a power supply applying a driving voltage fordriving the lamp unit is provided, and further includes a main inductorconnected to an input voltage, a main switching element of whichoperation is changed based on an externally applied control signal, amain diode connected to the lamp unit, an oscillator connected betweenthe main switching element and the main diode, and an auxiliaryswitching element connected to the oscillator and controlling theoscillator, wherein the main switching element and the main diode are tobe zero voltage switching or zero current switching in accordance withthe operations of the oscillator and the auxiliary switching element.

The oscillator may include a first capacitor connected in parallel tothe main switching element, a first inductor connected to the firstcapacitor, and a second inductor connected between the first capacitorand the auxiliary switching element.

In another embodiment of the present invention, a method of reducing aswitching loss in a first switching element of a DC-DC converterincludes diverting current away from the first switching element whenthe first switching element is being turned on or off, wherein the firstswitching element is at both zero current and zero voltage when beingturned on or off.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent by describingembodiments thereof in detail with reference to the accompanyingdrawings, in which:

FIG. 1 is an exploded perspective view of an exemplary embodiment of anLCD according to the present invention;

FIG. 2 is a block diagram of an exemplary part of the LCD shown in FIG.1;

FIG. 3 is an equivalent circuit diagram of an exemplary pixel of the LCDshown in FIG. 1;

FIG. 4 is a block diagram of an exemplary embodiment of a backlight unitaccording to the present invention;

FIG. 5 is a circuit diagram of an exemplary DC-DC converter shown inFIG. 4;

FIG. 6 illustrates waveforms detected at respective points of operationof the DC-DC converter shown in FIG. 4; and

FIGS. 7A to 7N are equivalent circuits of the DC-DC converter andillustrate current flows in respective operating modes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein.

In the drawings, the thickness of layers and regions are exaggerated forclarity. Like numerals refer to like elements throughout. It will beunderstood that when an element such as a layer, region, or substrate isreferred to as being “on” another element, it can be directly on theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly on” another element,there are no intervening elements present.

A display device, a driving apparatus of a light source for the displaydevice, and a DC-DC converter according to embodiments of the presentinvention will be described with reference to the accompanying drawings.

A liquid crystal display (“LCD”) according to an embodiment of thepresent invention is described in detail with reference to FIGS. 1-4.

FIG. 1 is an exploded perspective view of an exemplary embodiment of anLCD according to the present invention, FIG. 2 is a block diagram of anexemplary part of the LCD shown in FIG. 1, and FIG. 3 is an equivalentcircuit diagram of an exemplary pixel of the LCD shown in FIG. 1. FIG. 4is a block diagram of an exemplary backlight assembly according to anembodiment of the present invention.

Referring to FIG. 1, the LCD includes a display module 350 including adisplay unit 330 and a backlight assembly 900, and a pair of front andrear chassis 361 and 362 and a mold frame 364.

The display unit 330 includes an LC panel assembly 300, a plurality ofgate tape carrier packages (“TCPs”) or chip-on-film (“COF”) typepackages 410, a plurality of data TCPs 510 attached to the LC panelassembly 300, and a gate printed circuit board (“PCB”) 450 and a dataPCB 550 attached to the gate and the data TCPs 410 and 510,respectively.

The LC panel assembly 300 includes a lower panel 100, an upper panel200, and a liquid crystal layer 3 interposed therebetween as shown inFIG. 3.

The LC panel assembly 300 includes a plurality of display signal linesG1-Gn and D1-Dm and a plurality of pixels connected thereto and arrangedsubstantially in a matrix in circuital view.

The display signal lines G1-Gn and D1-Dm are disposed on the lower panel100 and include a plurality of gate lines G1-Gn transmitting gatesignals (also referred to as “scanning signals”), and a plurality ofdata lines D1-Dm transmitting data signals. The gate lines G1-Gn extendsubstantially in a row direction and are substantially parallel to eachother, while the data lines D1-Dm extend substantially in a columndirection and are substantially parallel to each other.

Each pixel includes a switching element Q connected to the displaysignal lines G1-Gn and D1-Dm, and an LC capacitor C_(LC) and a storagecapacitor C_(ST) connected to the switching element Q. The storagecapacitor C_(ST) may be omitted in certain embodiments.

The switching element Q, such as a TFT, is disposed on the lower panel100. The switching element Q has three terminals including a controlterminal connected to one of the gate lines G1-Gn, an input terminalconnected to one of the data lines D1-Dm, and an output terminalconnected to the LC capacitor C_(LC) and the storage capacitor C_(ST).

The LC capacitor C_(LC) includes a pixel electrode 190 provided on thelower panel 100 and a common electrode 270 provided on the upper panel200, as two terminals. The LC layer 3, disposed between the twoelectrodes 190, 270, functions as a dielectric of the LC capacitorC_(LC). The pixel electrode 190 is connected to the switching element Q,and the common electrode 270 is supplied with a common voltage Vcom andcovers an entire surface, or substantially the entire surface, of theupper panel 200. Alternatively, the common electrode 270 may be providedon the lower panel 100, and both electrodes 190 and 270 may have shapesof bars or stripes.

The storage capacitor C_(ST) is an auxiliary capacitor for the LCcapacitor C_(LC). The storage capacitor C_(ST) includes the pixelelectrode 190 and a separate signal line provided on the lower panel100. The storage capacitor C_(ST) also overlaps the pixel electrode 190via an insulator, and is supplied with a predetermined voltage such asthe common voltage Vcom. Alternatively, the storage capacitor C_(ST)includes the pixel electrode 190 and an adjacent gate line called aprevious gate line, which overlaps the pixel electrode 190 via aninsulator.

For color display, each pixel uniquely represents one of the red, blue,and green colors (i.e., spatial division) or each pixel sequentiallyrepresents the red, blue, and green colors in turn (i.e., temporaldivision) such that spatial or temporal sum of the primary colors arerecognized as a desired color. An example of a set of the colorsincludes red, green, and blue colors, however alternate color sets maybe employed. FIG. 3 shows an example of the spatial division in whicheach pixel includes a color filter 230 representing one of the red,blue, and green colors in an area of the upper panel 200 facing thepixel electrode 190. Alternatively, the color filter 230 is provided onor under the pixel electrode 190 on the lower panel 100.

As shown in FIGS. 1 and 2, the gate TCPs 410 are types of flexibleprinted circuit (“FPC”) films and are attached at an edge of the lowerpanel 100 of LC panel assembly 300. The gate TCPs 410 mount gateintegrated circuit (“IC”) chips forming a gate driver 400, and the dataTCPs 510 mount data IC chips forming a data driver 400. The gate driver400 and the data driver 500 are electrically connected to the gate linesG1-Gn and the data lines D1-Dm of the LC panel assembly 300 throughsignal lines (not shown) formed on the TCPs 410 and 510, respectively.

The gate driver 400 generates gate signals including a gate-on voltageVon and a gate-off voltage Voff for application to the gate lines G1-Gn,and the data driver 500 applies data voltages to the data lines D1-Dm.

Alternatively, the driving IC chips for the gate driver 400 and the datadriver 500 may be directly mounted on the panel assembly without TCPs,which is called a “chip on glass” (“COG”) type of mounting. The gatedriver 400 or the data driver 500 may be formed on the LC panel assembly300 in company with the switching elements Q, the display signal linesG1-Gn, and the data lines D1-Dm.

The gate PCB 450 is attached to the TCPs 410 in a longitudinal directionalong an edge of the lower panel 100, and the data PCB 550 is attachedto the TCPs 510 in a longitudinal direction along another edge of thelower panel 100. As shown in FIG. 2, the gate PCB 450 and the data PCB550 mount a gray voltage generator 800 and a signal controller 600 aswell as signal lines (not shown). The gray voltage generator 800generates two sets of gray voltages related to the transmittance of thepixels, and applies gray voltages to be selected as the data voltages tothe data driver 500.

The signal controller 600 controls the drivers 400 and 500, sendssignals to the backlight assembly 900, etc.

As shown in FIGS. 1, 2, and 4, the backlight assembly 900 includes alamp unit 960 housed in the rear chassis 362 and fixed thereto anddisposed on the LC panel assembly 300, a plurality of optical components910 disposed between the LC panel assembly 300 and the lamp unit 960 foradjusting light emitting form the lamp unit 960, and a power supply 950applying a supply voltage to the lamp unit 960. The backlight assembly900 also includes a supporting frame 905 housed in the rear chassis 362to fix the lamp unit 960. The supporting frame 905 is fixed to the moldframe 364.

The lamp unit 960 includes a plurality of PCBs 962 that mount aplurality of LEDs 961, respectively, and radiant heat members 963attached to the PCBs 962 to radiate heat. The radiant heat members 963are preferably made of heat conduction materials. Each PCB 962 isarranged horizontally along a longitudinal axis, and mounts in turn red,green, and blue LEDs 961. The number of green LEDs may be larger thanthe number of red and blue LEDs, for example, preferably by about twotimes. However, the number of the LEDs may be changed if necessary, andalternate arrangements of LEDs are within the scope of theseembodiments.

One or more polarizers (not shown) for polarizing the light from thelamp unit 960 are attached to the outer surfaces of the panels 100 and200.

The optical components 910 include a reflector 904, a light guide plate903, a spread plate 902, such as a diffusing plate, and at least oneoptical sheet 901.

The reflector 904 is disposed between the LC panel assembly 300 and thelamp unit 960. The reflector 904 has a plurality of light emitting holesof a predetermined size which are arranged at a predetermined intervalfor allowing the LEDs 961 to pass there through. The reflector 904 alsoreflects light not passing through the light emitting holes in adownward direction.

The light guide plate 903 is disposed over the reflector 904, such asbetween the reflector 904 and the spread plate 902, and has lightblocking films formed on portions facing the respective LEDs 961. Thelight guide plate 903 uniformly maintains the intensity of light fromthe LEDs 961. The spread plate 902 guides and spreads light from thelight guide plate 903 to the LC panel assembly 300.

The optical sheet or sheets 901 enhances luminance characteristics.

In FIG. 1, each light emitting hole formed on the reflector 904 has acircular shape such that the corresponding LED is projected through thehole. Although not illustrated, the reflector may alternatively includealternate shapes of light emitting holes such as a rectangular shape ora slit shape that are adapted to project a predetermined number of theLEDs 961. The internal lateral sides of the supporting frame 905 areinclined relative to an upper surface thereof, to reflect the light fromthe lamp unit 960 toward an upward direction towards the LC panelassembly 300.

As shown in FIG. 4, the power supply 950 includes a line filter 910, abridge rectifier 920 connected to the line filter 910, a smoother 930connected to the bridge rectifier 920, the bridge rectifier 920connected between the line filter 910 and the smoother 930, and a DC-DCconverter 940 connected to the smoother 930, the smoother 930 connectedbetween the bridge rectifier 920 and the DC-DC converter 940. The powersupply 950 is supplied with an AC voltage of about 90V to 220V, which isfirst received by the line filter 910.

Although not shown in FIG. 1, an upper case and a lower case aredisposed on the front chassis 361 and the rear chassis 362,respectively, and combine with each other to substantially complete theLCD.

Now, the operation of the LCD will be described in detail with referenceto FIGS. 1 to 4.

Referring to FIG. 2, the signal controller 600 is supplied with inputred, green, and blue image signals R, G, and B and input control signalscontrolling the display thereof such as a vertical synchronizationsignal Vsync, a horizontal synchronization signal Hsync, a main clockMCLK, and a data enable signal DE from an external graphics controller(not shown). The signal controller 600 generates gate control signalsCONT1, data control signals CONT2, and backlight control signals CONT3and processing the image signals R, G, and B to be suitable for theoperation of the panel assembly 300 on the basis of the input controlsignals and the input image signals R, G, and B. The signal controller600 then provides the gate control signals CONT1 to the gate driver 400,the processed image signals DAT and the data control signals CONT2 tothe data driver 500, and the backlight control signals CONT3 to thebacklight assembly 900.

The gate control signals CONT1 include a scanning start signal STVhaving instructions to start scanning and at least one clock signal forcontrolling the output time of the gate-on voltage Von. The gate controlsignals CONT1 may further include an output enable signal OE fordefining the duration of the gate-on voltage Von.

The data control signals CONT2 include a horizontal synchronizationstart signal STH for informing the data driver 500 of a start of datatransmission for a group of pixels, a load signal LOAD havinginstructions to apply the data voltages to the data lines D₁-D_(m), anda data clock signal HCLK. The data control signal CONT2 may furtherinclude an inversion signal RVS for reversing the polarity of the datavoltages with respect to the common voltage Vcom.

The backlight control signals CONT3 include a plurality of controlsignals such as pulse width modulation (“PWM”) signals controlling theLEDs 961 of the lamp unit 960 and controlling the DC-DC converter 940 ofthe power supply 950.

In response to the data control signals CONT2 from the signal controller600, the data driver 500 receives a packet of the image data DAT, theprocessed image signals, for the group of pixels from the signalcontroller 600, converts the image data DAT into analog data voltagesselected from the gray voltages supplied from the gray voltage generator800, and applies the data voltages to the data lines D₁-D_(m).

The gate driver 400 applies the gate-on voltage Von to the gate linesG₁-G_(n) in response to the gate control signals CONT1 from the signalcontroller 600, thereby turning on the switching elements Q connectedthereto. The data voltages applied to the data lines D₁-D_(m) aresupplied to the pixels through the activated switching elements Q.

The difference between the data voltage and the common voltage Vcomapplied to a pixel is expressed as a charged voltage of the LC capacitorC_(LC), i.e., a pixel voltage. The LC molecules have orientationsdepending on the magnitude of the pixel voltage.

As shown in FIG. 4, the DC-DC converter 940 of the power supply 950 inthe backlight assembly 900 supplying driving voltages for the lamp unit960 filters noise contained in an AC input voltage from the outside byusing the line filter 910 to apply the AC input voltage to the bridgerectifier 920. The line filter has a turn ratio of “1” of a primary coil(not shown) and a secondary coil (not shown) such that loss of an outputvoltage with respect to the input voltage is not generated.

The bridge rectifier 920 half wave rectifies the AC input voltage fromthe line filter 910 to convert a DC voltage and applies the converted DCvoltage to the smoother 930.

The DC voltage received from the bridge rectifier 920 has ripplessmoothed by the smoother 930. The smoother 930 applies the smoothed DCvoltage to the DC-DC converter 940.

The DC-DC converter 940 boosts the DC voltage inputted from the smoother930 to a predetermined magnitude for driving the lamp unit 960 based onthe backlight control signals CONT3, and outputs the boosted DC voltageto the lamp unit 960. The operations of the DC-DC converter 940 will befurther described below.

The lamp unit 960 drives, based on the backlight control signals CONT3and the DC voltage from the DC-DC converter 940 of the power supply 950,to turn on or turn off the LEDs 961. By a mix of the light of the red,green, and blue colors from the red, green, and blue LEDs 961, whitelight is emitted from the lamp unit 960.

The light from the backlight assembly 900 passes through the LC layer 3and experiences a change of its polarization. The change of thepolarization is converted into that of the light transmittance by thepolarizers formed on exterior surfaces of the panels 100, 200.

By repeating this procedure by a unit of the horizontal period (which isdenoted by “1H” and is equal to one period of the horizontalsynchronization signal Hsync and the data enable signal DE), all gatelines G₁-G_(n) are sequentially supplied with the gate-on voltage Vonduring a frame, thereby applying the data voltages to all pixels. Whenthe next frame starts after finishing one frame, the inversion controlsignal RVS, part of the data control signals CONT2, applied to the datadriver 500 is controlled such that the polarity of the data voltages isreversed (which is referred to as “frame inversion”). The inversioncontrol signal RVS may also be controlled such that the polarity of thedata voltages flowing in a data line in one frame are reversed (forexample, line inversion and dot inversion), or the polarity of the datavoltages in one packet are reversed (for example, column inversion anddot inversion).

Now, a DC-DC converter 940 according to an embodiment of the presentinvention will be described in detail with reference to FIGS. 4-6 and 7Ato 7N.

FIG. 5 is a circuit diagram of a DC-DC converter 940 shown in FIG. 4,FIG. 6 illustrates waveforms detected at respective points of the DC-DCconverter 940 shown in FIG. 4, and FIGS. 7A to 7N are equivalentcircuits of the DC-DC converter 940 and illustrate current flows inrespective operating modes.

As shown in FIG. 5, the DC-DC converter 940 is a boost DC-DC converterand includes an inductor Lf supplied with a DC input voltage Vin fromthe smoother 930, a main switching element 5 connected in parallelbetween the inductor Lf and the input voltage Vin, and a diode Dsconnected in parallel to both terminals of the main switching element 5in a reverse direction from the main switching element S. That is,current flowing through the diode Ds flows in an opposite direction fromcurrent flowing through the main switching element 5, and thus, currentdoes not flow through both the diode Ds and the main switching element 5at the same time. The DC-DC converter 940 also includes a parasiticcapacitor Cp connected in parallel to the diode Ds, a capacitor Crconnected to the main inductor Lf, an inductor Lr1 connected in seriesto the capacitor Cr, a diode Da connected to the inductor Lr1 and theinput voltage Vin in a forward direction from the inductor Lr1, aninductor Lr2 connected to the inductor Lf, an auxiliary switchingelement Sa connected between the inductors Lr2 and Lr1, a main diode Drconnected to the main inductor Lf in a forward direction from the maininductor Lf, a diode Dc commonly connected to the inductor Lr2 and theauxiliary switching element Sa in a forward direction from the inductorLr2, and a capacitor Co connected to the diodes Dr and Dc and the inputvoltage Vin. The main switching element 5 is a bipolar junctiontransistor (“BJT”), and the auxiliary switching element Sa is amajority-carrier metal oxide silicon field effect transistor (“MOSFET”).However, the types of the switching elements may be changed, and suchalternatives are within the scope of these embodiments.

The switching element 5, the inductor Lf, and the diode Dr are a mainswitching element, a main inductor, and a main diode for DC-DCconverting, respectively. Specifically, the main inductor Lf is an inputfiltering inductor. The diode Ds and the capacitor Cp are a parasiticdiode and a parasitic capacitor generated in the main switching element5, respectively.

The capacitor Cr and the inductors Lr1 and Lr2 are elements foroscillating. The auxiliary switching element Sa is an auxiliaryswitching element for DC-DC converting. The diodes Da and Dc areauxiliary diodes also for DC-DC converting, and in particular the diodeDc is a diode for clamping.

The capacitor Co is an output capacitor.

The operations of the DC-DC converter 940 according to an embodiment ofthe present invention will now be described in detail.

For analyzing the operations of the DC-DC converter 940, it is assumedthat an input current Ii and an output voltage V0 are constant for anoperating period due to enough large capacity of the inductor Lf and thecapacitor Co, and therefore main inductor Lf and output capacitor Co arenot specifically illustrated in the following figures. Furthermore, foranalyzing the operations of the DC-DC converter 940, it is assumed thatall elements are ideal.

For description purposes, the DC-DC converter 940 has a total of 14modes for an operating period according to an embodiment of the presentinvention, of course, the operating period may be divided into analternate number of modes.

Mode 1 (t0-t1)

As shown in waveforms (a) and (b) of FIG. 6, the main switching elementS and the auxiliary switching element Sa are respectively supplied witha signal with a low state, to make the switching elements S and Sa turnoff.

The input voltage Vin is transmitted to a load, that is, the lamp unit960, through the diode Dr as indicated in FIG. 7A. The current IDr ofthe diode Dr is equivalent to the input current Ii as demonstrated bywaveform (h) of FIG. 6. At this time, the capacitor Cr is charged to amaximum voltage Vcrmax, as demonstrated by waveform (k) of FIG. 6.

Mode 2 (t1-t2)

At this time, the main switching element S is still turned off and theauxiliary switching element Sa is turned on as demonstrated by thewaveforms (a) and (b) of FIG. 6. Accordingly, as shown in FIG. 7B, theturned on auxiliary switching element Sa and the capacitor Cr and theinductors Lr1 and Lr2 for oscillating form a closed circuit. A currentflows through the auxiliary element Sa and the diode Da.

Accordingly, the current flows through the closed circuit by theoscillation generated by the capacitor Cr and the inductors Lr1 and Lr2such that a voltage Vcr charged in the capacitor Cr comes to graduallydecrease as demonstrated by waveform (k) of FIG. 6, and currents ILr1and ILr2 flowing through the inductors Lr1 and Lr2 come to graduallyincrease, as demonstrated by waveforms (i) and (j). At this time, sincethe currents ILr1 and ILr2 flow in opposite directions from each other,the current ILr1 has a (−) direction.

By the oscillation, the diode Da is turned on, and thereby a currentflowing through the turned-on diode Da comes to gradually increase byadding the current flowing in the closed circuit formed by the capacitorCr and the inductors Lr1 and Lr2 to the input current Ii. However, sincethe current IDr flowing through the diode Dr also flows through theclosed circuit, the current IDr comes to gradually decrease from amaximum equivalent to the input current Ii to a minimum current asdemonstrated by waveform (h) of FIG. 6.

Mode 3 (t2-t3)

At this time, the main switching element 5 is turned off and theauxiliary switching element Sa is turned on as shown in FIG. 7C. When amagnitude of the current flowing through the diode Da reaches amagnitude of the input current Ii, the diode Dr is turned off such thatthe current flowing through the diode Dr is cut off and the diode Dr isturned off by zero voltage switching, as demonstrated by waveform (h) ofFIG. 6. At this time, the parasitic capacitor Cp of the main switchingelement 5 comes to discharge the charges after the charging such thatthe oscillation by the capacitor Cp and the inductor Lr2 begins. Thus,the current flowing through the diode Da passes through the parasiticcapacitor Cp.

Mode 4 (t3-t4)

When the main switching element 5 is turned off and the auxiliaryswitching element Sa is turned on, as shown in FIG. 7D, the dischargingof the capacitor Cp is maintained so that the current ILr2 flowing tothe inductor Lr2 reaches a maximum as demonstrated by waveform (i) ofFIG. 6. At this time, since the current flowing via the diode Da islarger than the input current Ii, the diode Ds is turned on to flow acurrent. When the current ILr2 flowing through the inductor Lr2 reachesthe maximum, all the energy charged in the capacitor Cp is moved intothe inductor Lr2. Thus, the flowing of the current is moved into thediode Ds from the capacitor Cp, to turn on the diode Ds. At this time,the current flowing via the diode Da is gradually decreased, but thediode Ds maintains the turned-on state until a magnitude of the currentflowing via the diode Da is less than that of the input current Ii.

The current flow is diverted away from the main switching element S andsent through the diode Ds while the main switching element S is beingturned on. Accordingly, for the turn on of the diode Ds, when the mainswitching element S is turned on as demonstrated by waveform (a) of FIG.6, the zero voltage switching and the zero current switching aresimultaneously generated.

Mode 5 (t4-t5):

At this time, the main switching element S is turned on and theauxiliary switching element Sa is turned off. In the mode 4, when allthe energy charged in the capacitor Cr is moved into the inductor Lr2,the current flowing through capacitor Cr is reversed to graduallyincrease the charge voltage Vcr of the capacitor Cr as demonstrated bywaveform (k) of FIG. 6. In this state, when the auxiliary switchingelement Sa is turned off, as shown in FIG. 7E, the current ILr2 flowingthrough the inductor Lr2 flows toward the diode Dc to be applied to theload, which is the lamp unit 960. Thus, the current ILr2 flowing throughthe inductor Lr2 begins to decrease as demonstrated by waveform (i) ofFIG. 6.

In addition, the current ILr1 flows toward the diode Da, and thereforegradually decreases as demonstrated by waveform (j) of FIG. 6. Aspreviously described with respect to mode 4, as long as the magnitude ofthe current flowing through the diode Da does not decrease to that ofthe input current Ii, the diode Ds maintains a turned-on state and thecurrent continues to flow through the diode Ds. Thus the main switchingelement S continuously maintains the zero voltage switching state.

Mode 6 (t5-t6)

At this time, the main switching element S maintains the turned-on stateand the auxiliary switching element Sa maintains the turned-off state.The sum of the currents ILr1 and ILr2 flowing through the inductors Lr1and Lr2, respectively gradually being decreased from “t4”, is less thanthe magnitude of the input current Ii at “t5.” Since the current flowingvia the diode Da is no longer larger than the input current Ii, thediode Ds is no longer turned on to flow a current, and the currentinstead flows through the main switching element S in the oppositedirection. The current increase through the main switching element S isdemonstrated by waveform (d) of FIG. 6. Thus, the auxiliary switchingelement Sa maintains the turned-off state such that a voltage applied toboth terminals of the auxiliary switching element Sa maintains an outputvoltage Vo. This mode is illustrated in FIG. 7F.

Mode 7 (t6-t7):

At this time, the main switching element S maintains the turned-on stateand the auxiliary switching element Sa maintains the turned-off state.The magnitude of the current Is flowing through the main switchingelement S in the mode 6 reaches the magnitude of the input current Ii asdemonstrated by waveform (d) of FIG. 6, and the equivalent circuit ofthe DC-DC converter 940 is as shown in FIG. 7G. That is, only onecurrent flows through the main switching element S. Thus, the energyfrom the input voltage Vin is continuously charged in the main inductorLf, as shown in FIG. 5, and the energy charged in the output capacitorCo, also as shown in FIG. 5, is transmitted into the lamp unit 960.

Mode 8 (t7-t8):

At this time, the main switching element S and the auxiliary switchingelement Sa are both turned on, as demonstrated by waveforms (a) and (b)in FIG. 6. By turning on the auxiliary switching element Sa, anoscillation is generated by the capacitor Cr and the inductors Lr1 andLr2 as shown in FIG. 7H and the current Is a flowing through theauxiliary switching element Sa increases as demonstrated by waveform (f)of FIG. 6. Thus, the currents ILr1 and ILr2 begin to increase, asdemonstrated by waveforms (i) and (j) of FIG. 6, until a voltage VCrcharged in the capacitor Cr is entirely discharged, as demonstrated bywaveform (k) of FIG. 6. In this case, the diode Da is not turned off yetdue to voltage drop generated by the capacitor Cr and the inductor Lr1.

At this time, the charged voltage of the capacitor Cr is changed suchthat the auxiliary switching element Sa is in a zero current switchingstate.

Mode 9 (t8-t9)

At this time, the main switching element 5 and the auxiliary switchingelement Sa are maintained in the turned-on state. The currents ILr1 andILr2 of the inductors Lr1 and Lr2 reach the maximum as shown inwaveforms (i) and (j) at “t8”, and thereby the diode Da is turned on.Thus, since the current flowing through the diode Da increases, thecurrent Is flowing through the main switching element 5 decreases asdemonstrated by waveform (d) of FIG. 6. As shown in FIG. 7I, the inputcurrent Ii begins to flow though the inductor Lr2 such that anoscillation among the inductors Lr1 and Lr2 and the capacitor Cr isgenerated. Thus, a flowing direction of the current ILr1 flowing throughthe inductor Lr1 is changed as demonstrated by waveform (j) of FIG. 6.

Mode 10 (t9-t10)

At this time, the main switching element 5 and the auxiliary switchingelement Sa are maintained in the turned-on state. As shown in FIG. 7J,the capacitor Cr begins to charge such that the charged voltage Vcrincreases as demonstrated by waveform (k) of FIG. 6, and the flowingdirection of the current ILr1 flowing through the inductor Lr1 ischanged as demonstrated by waveform (j) of FIG. 6. Until the magnitudeof the current of the diode Da begins to increase in the mode 9 to reachthat of the input current Ii, the current continuously flows through themain switching element 5, but the current Is is continuously decreased,as demonstrated by waveform (d) of FIG. 6, as the increment of thecurrent flowing through the diode Da. A magnitude of the current ILr1flowing through the inductor Lr1 is less than that of the input currentIi, as demonstrated by waveform (j) of FIG. 6.

Mode 11 (t10-t11)

The main switching element 5 is turned off and the auxiliary switchingelement Sa is maintained in the turned-on state, as demonstrated bywaveforms (a) and (b) of FIG. 6. When the magnitude of the currentflowing through the diode Da is larger than that of the input current Iithe diode Ds is turned on as shown in FIG. 7K, to flow the remainingcurrent which did not flow toward the input voltage Vin. At this time,the turned-on operation of the diode Ds is zero current switching. Forthe turn on of the diode Ds, the main switching element 5 is turned offto generate zero voltage switching. Accordingly, the zero voltageswitching and the zero current switching are simultaneously generated.

Mode 12 (t11-t12)

At this time, as shown in FIG. 7L, the main switching element 5maintains the turned-off state and the auxiliary switching element Sa isalso turned off, as demonstrated by waveforms (a) and (b) of FIG. 6. Bythe turn-off of the auxiliary switching element Sa, the energy chargedin the inductor Lr2 is transmitted into the lamp unit 960 via the diodeDc.

Thus, the current ILr2 flowing through the inductor Lr2 is graduallydecreased as demonstrated by waveform (i) of FIG. 6. In addition, by theenergy transmission to the load, the lamp unit 960, the current flowingthrough the diode Ds is gradually decreased and finally does not flow,as illustrated in FIG. 7L. However, the current through the capacitorCr, the inductor Lr1, and the diode Da is maintained and the mainswitching element 5 maintains the zero voltage switching state.Moreover, the voltage Vsa applied to both terminals of the auxiliaryswitching element Sa maintains the output voltage V0, as demonstrated bywaveform (e) of FIG. 6.

Mode 13 (t12-t13)

At this time, as shown in FIG. 7M, the main switching element 5 and theauxiliary switching element Sa maintain the turned-off state. The energywhich began to be transmitted toward the load is all transmitted intothe load such that the magnitude of the current ILr2 becomes “0” asdemonstrated by waveform (i) of FIG. 6 to turn off the diode Dc.

Thus, a portion of the input current Ii flows through the capacitor Cpsuch that the capacitor Cp is charged to a magnitude of the outputvoltage V0. At this time, although a current flows through the capacitorCr and the inductor Lr1 due to the increment of the current ILr1, thepotential difference between both terminals of the diode Da does notbecome a zero voltage.

Mode 14 (t13-t14):

At this time, as shown in FIG. 7N, the main switching element S and theauxiliary switching element Sa continuously maintain the turned-offstate. When a magnitude of the charged voltage in capacitor Cr reachesoutput voltage V0 as demonstrated by waveform (k) of FIG. 6, the inputcurrent Ii flows through the diode Dr, as demonstrated by waveform (h)of FIG. 6, as well as the capacitor Cr such that the diode Dr is turnedon, and thereby a voltage applied to both terminals of the diode Dr is azero voltage. as demonstrated by waveform (g) of FIG. 6. Moreover, thedifference between the charged voltage in the capacitor Cr and a voltageapplied to both terminals of the inductor Lr1 define the magnitude ofthe output voltage V0.

As shown in FIG. 6, a magnitude of the current IDr, as demonstrated bythe waveform (h), flowing through the diode Dr at “t14” reaches that ofthe input current Ii at “t0,” and a charged voltage in the capacitor Crreaches the maximum Vcrmax, as demonstrated by the waveform (k) of FIG.6. The current ILr2 is clamped, as demonstrated by waveform (i) of FIG.6, by the diode Dc.

According to the present invention, when the main switching element andthe main diode are turned on or off, a zero voltage switching state or azero current switching state comes about by the operations of theauxiliary switching element of the majority-carrier semiconductorelements to decrease the switching loss of the switching elements.Furthermore, in one exemplary embodiment, when the main switchingelement is turned on or off, zero voltage switching and zero currentswitching are simultaneously generated, thus preventing a switchingloss. Similarly, when the main diode is turned on or off, a zero voltageis applied to both terminals of the main diode. In addition, the presentinvention may be applicable to a switching element of theminority-carrier semiconductor elements.

Moreover, current and voltage stress is decreased, and thereby theoperational efficiency of the DC-DC converter improves.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptsherein taught which may appear to those skilled in the present art willstill fall within the spirit and scope of the present invention, asdefined in the appended claims. Moreover, the use of the terms first,second, etc. do not denote any order or importance, but rather the termsfirst, second, etc. are used to distinguish one element from another.Furthermore, the use of the terms a, an, etc. do not denote a limitationof quantity, but rather denote the presence of at least one of thereferenced item.

1-8. (canceled)
 9. A driving apparatus of a display device including alight source unit having a plurality of light sources, the drivingapparatus comprising: a power supply applying a driving voltage fordriving the light sources; and a control signal outputting a firstcontrol signal controlling the power supply and a second control signalcontrolling the light sources, the driving apparatus controllingoperations of the light sources by the driving voltage based on thesecond control signal, wherein the power supply comprises a DC-DCconverter having a main switching element, operation of the mainswitching element changed by the first control signal, a main diodeconnected to the light source unit, an oscillator connected between themain switching element and the main diode, and an auxiliary switchingelement connected to the oscillator and controlling operation of theoscillator, the DC-DC converter boosting an input DC voltage to thedriving voltage by a predetermined magnitude and applying the boosteddriving voltage to the light source unit, wherein the DC-DC converter isto be zero voltage switching or zero current switching of the mainswitching element and the main diode in accordance with operations ofthe oscillator and the auxiliary switching element.
 10. The drivingapparatus of claim 9, wherein the oscillator comprises: a firstcapacitor connected in parallel to the main switching element; a firstinductor connected to the first capacitor; and a second inductorconnected between the first capacitor and the auxiliary switchingelement.
 11. The driving apparatus of claim 9, further comprising in thepower supply a rectifier changing an externally supplied AC voltage intoa DC voltage and a smoother smoothing the DC voltage from the rectifierand outputting the smoothed DC voltage as the input DC voltage to theDC-DC converter.
 12. The driving apparatus of claim 11, wherein thepower supply further comprises a filter filtering noises included in theAC voltage.
 13. The driving apparatus of claim 10, wherein the DC-DCconverter further comprises: a third inductor connected to the input DCvoltage; a first diode connected to the auxiliary switching element andthe first inductor; a second diode connected to the second inductor; anda second capacitor connected to the main diode and the second diode. 14.The driving apparatus of claim 9, wherein the main switching element isa semiconductor device of a type different from that of the auxiliaryswitching element.
 15. The driving apparatus of claim 14, wherein theauxiliary switching element is a metal oxide silicon field effecttransistor.
 16. The driving apparatus of claim 9, wherein the lightsources comprise light emitting diodes of at least one red, green, andblue colors. 17-22. (canceled)
 23. A display device including aplurality of pixels, a lamp unit emitting light based on a lamp controlsignal, and a power supply applying a driving voltage for driving thelamp unit, the display device comprising: a main inductor connected toan input voltage; a main switching element, the main switching elementchanging operation based on an externally applied control signal; a maindiode connected to the lamp unit; an oscillator connected between themain switching element and the main diode; and an auxiliary switchingelement connected to the oscillator and controlling the oscillator,wherein the main switching element and the main diode are to be zerovoltage switching or zero current switching in accordance withoperations of the oscillator and the auxiliary switching element. 24.The display device of claim 23, wherein the oscillator comprises: afirst capacitor connected in parallel to the main switching element; afirst inductor connected to the first capacitor; and a second inductorconnected between the first capacitor and the auxiliary switchingelement. 25-27. (canceled)